The type of drilling apparatuses described above have been used for a long time and for example in patent publication FI 75650 there has been presented a boring tool, which is meant for boring and/or hammer drilling, to be used in connection with a drill rod unit placed inside a core pipe. The boring tool to be attached at the front end of the drill rod unit has a center being provided with a cutting unit, and an eccentric reaming drill, being placed behind the center drill, the reaming drill having also a cutting unit. The reaming drill moves with respect to the center drill between a drilling position, in which it is positioned to the side in front of the core pipe, and a return position, in which it is withdrawn in radial direction inside the core pipe. Deviating from earlier solutions, in which the center drill is in most cases provided with four cutting parts directed radially and being made of hard metal, the reamer for its part comprising either one or two radially directed cutting parts made of hard metal, in the solution according to the publication in question, the cutting parts are replaced by bit parts being arranged in a certain manner. With the solution presented in the publication in question such constructions of the center drill and the reaming drill have been, desired that the operating time of the boring tool will be as long as possible.
It is thus characteristic for use of the type of drilling apparatuses described above that the hole to be drilled to the ground is reamed by an eccentrically operating reaming drill. This leads to most differing kinds of problems. First of all the drilling motion of the reaming drill taking place eccentrically causes high torsion stress to the drill rod unit. In this case it is not possible to take optimally advantage of the operating efficiency of the drilling device due to high breakage risk of the drill rod unit, particularly breaking off thereof. On the other hand, a problem in practice in a drilling situation is often the returning of the reaming drill to its return position, which is due to ground substance, being collected between the reaming drill and the center drill. In order to get the reaming drill back to its return position, usually a repeated back and forth movement of the drilling device is required. Naturally when e.g. a too high operating power is being used or when the materials of the drilling device parts have a lowered fatigue resistance, there is a high risk for breakage of the drill rod unit.
On the other hand e.g. in Finnish patent number 94891 there has been presented a boring tool, which has two or three blocks moving in respect with each other in the same plane, the blocks forming a drill bit of the boring tool. The boring tool in question is usable as described above in a way that in a drilling situation the bit blocks are in an ejected position, in which case the bit parts placed therein drill a hole that is essentially larger than the casing part. After a drilling situation the casing part may be left in the hole and the drill rod unit inside the same with its bit blocks can be removed as a whole from the hole by moving the bit blocks with respect to each other in a way that they get withdrawn to a form that is smaller than the free internal diameter of the casing part. In the solution in question, the bit blocks are arranged movable in respect with each other by coupling each one thereof by a single guiding pin with a guide part supporting the bit blocks, which pins depending on the rotation direction of the guide part bring about protrusion of the bit blocks or the same getting together in cooperation with guide grooves in the guide part. In the patent in question several differing solutions have been presented in order to couple these guiding pins with the bit blocks. The biggest problem in practice of these types of solutions is, however, the high stresses directed to the guiding pins, in which case particularly when drilling stony ground, the durability of the guide pins is put to a tough test, because very high torsion stresses as well as torque loads are directed to the guiding pins. This is why the functionality of the guiding pins particularly in drilling to be performed under demanding circumstances is very unreliable, because when drilling e.g. rock a continuous fatigue load is directed to the pins, which furthermore weakens tensile properties thereof in longer lasting use.
Furthermore for example from patent documents CH 622312, U.S. Pat. No. 5,361,859, U.S. Pat. No. 3,365,010, GB 2310229 and GB 973790 it is known that the second drill bit arrangement is arranged by two or more bit pieces that are movable in radial direction in slide grooves that pass wide open on the drilling surface of the drilling means, and, in which the drilling organs are arranged when viewed in longitudinal direction of the drilling device on the front surface thereof. It is common to all of the solutions described above that the slide grooves therein, being inclined towards the drilling head of the drilling device when viewed in a longitudinal cross section, whereby the said bit pieces are arranged movable in the slide grooves e.g. by influence of a flushing medium being used in the drilling or by influence of a force directed thereto when the drilling head of the drilling device is being pushed against the ground. The return movement of the bit pieces in question for reducing the drilling head is carried out e.g. by spring force or by gravitation when the drilling head is being lifted upwards inside the casing part. With the type of solutions in question it is not in practice possible to e.g. lift the casing part upwards in a drilled hole, which is necessary in certain occasions.
A problem of the type of drilling heads described above is the very strongly conical drilling surfaces, which is why wearing out of the bits takes place unevenly, whereby a part of the drill bits wear out prematurely. On the other hand a further difficult problem in practice related to the type of implementations in question is the fact that by the solutions utilized therein returning of the bit pieces to the reduced position of the drilling head is practically uncertain, and is not even possible under all circumstances, which is why the drilling head can necessarily not be removed from a drilled hole without getting broken. In addition, the type of drilling device in question may “get stuck”, which requires reciprocating use in order to get the drill bit released, which causes the same problems that have been explained above about the use of eccentrical drills. Due to the above, with the types of implementations in question, it is not possible to carry out drilling in practice in an adequately reliable and long-lasting manner.